1. Field of the Invention
The present invention relates to a process for treating wastewaters and circulation waters in papermaking, in deinking and in pulp bleaching by adsorption of water-soluble anionic compounds and compounds dispersed in water from the wastewaters and circulation waters over a finely divided adsorbent and the use of finely divided, water-soluble, only slightly swellable polymers, which contain polymerized vinylamine units, as adsorbents.
2. Description of the Background
Since the water circulations in the paper mills become increasingly concentrated, anionic compounds accumulate in the recycled water and very adversely affect the efficiency of cationic polymeric process chemicals in the drainage of paper stock and the retention of fillers and fibers. In practice, water which is completely or at least partly recycled to the paper machine is used for the production of the paper stock. This is either clarified or unclarified white water or a mixture of such water qualities. The recycled water contains smaller or larger amounts of interfering substances which are known to have a very adverse effect on the efficiency of cationic drainages and retention aids as well as other process chemicals. The content of the interfering substances in the paper stock can be characterized, for example, by the cumulative parameter of chemical oxygen demand (COD). The COD of such paper stock are, for example, 300-30,000, in general 1000-20,000 mg of oxygen/kg of the aqueous phase of the paper stock. These amounts of interfering substances greatly impair the efficiency of cationic process chemicals as long as they are used in papermaking in the absence of fixing agents.
To remove the interfering substances from the white water, for example, drainage of these paper stocks is additionally carried out in the presence of a fixing agent. The fixing agents used are, for example, condensates of dicyandiamide and formaldehyde or condensates of dimethylamine and epichlorhydrin (cf. Tappi Journal 8 1988, 131). EP-A-0 438 707 discloses a process in which water-soluble hydrolyzed homo- and/or copolymers of N-vinylformamide with a degree of hydrolysis of at least 60% are used as fixing agents for interfering substances.
EP-A-0 649 941 describes a process in which water-soluble polymers containing vinylamine units are used in order to remove undesired impurities, for example resins, from the white water. U.S. Pat. No. 5435921 discloses the use of such polymers in combination with other polymers for decolorizing the paper stock.
Wochenblatt fur Papierfabrikation 16 (1990), 709, and the literature cited there describe processes in which the dissolved interfering substances are coagulated, for example by means of positively charged polymers, flocculated and finally removed from the water by sedimentation, flotation or filtration. Wochenblatt fur Papierfabrikation 7 (1981), 225 discloses a corresponding process for treating water in the deinking process. A process for removing ligninsulfonates by precipitation with water-soluble cationic oligomers and polymers is described in EP-A-0 049 831. However, this process requires very complicated working-up of the precipitated material with the use of chlorinated organic solvents, so that it is not feasible for economic and especially ecological reasons.
The use of solids having a large surface area as adsorbents, for example active carbon, is also known. Macroporous ion exchangers and adsorber resins are also suitable. Their use for treating pulp bleaching liquors, paper machine circulation waters and papermill wastewaters is described in Wochenblatt fur Papierfabrikation 7 (1981), 205.
However, none of these adsorption processes have become established in the pulp industry and in particular in the paper industry. This is due on the one hand to the high costs of the adsorbents used, but also to their high selectivity, their insufficient capacity for the large amounts of different dissolved anionic substances and dispersed anionic solid particles and the relatively slow adsorption processes which are determined, for example, by the penetration rate of the dissolved or dispersed, oligomeric and polymeric compounds. In the process in which the dissolved anionic substances are coagulated in the circulation water, the disadvantages likewise predominate over the advantages. Although the reaction between coagulant and material to be precipitated is very rapid owing to the reaction in the homogeneous phase, the consumption of expensive chemicals is very high because they have to be used in stoichiometric amounts and finally cannot be recovered. The precipitated coagulum must furthermore be disposed. Another disadvantage is the highly complicated logistics of at least two different process chemicals and of course the expensive apparatuses for separating off the coagulum in the flotation plants and possibly for thickening the flotation liquors.
EP-A-0667 874 discloses insoluble, only slightly swellable amino-containing polymers which are obtainable, for example, from popcorn polymers which contain vinylformamide units by eliminating formyl groups from the polymerized vinylformamide units with formation of vinylamine units. The amino-containing popcorn polymers are used, for example, as ion exchangers or adsorber resins for metal ions.